The medicines, which help unleash the immune system on cancer cells, were tested in different categories of patients with advanced lung cancer, so the results aren't directly comparable. They were shared at the American Association for Cancer Research conference, or AACR, in Chicago.

Merck's drug, Keytruda, reduced the risk of death in the trial by 51 percent when combined with chemotherapies, versus chemo alone.

John Tlumacki | The Boston Globe | Getty Images

A research lab at Merck

Bristol's drug, Opdivo, along with another of its medicines, Yervoy, reduced the risk of cancer progression or death in its study by 42 percent when compared with chemo.

Analysts at Cowen Research had been expecting a reduction in these measures of at least 30 percent.

Leaders of both trials declared the results would change the way doctors prescribe medicines for lung cancer.

"Results from KEYNOTE-189 are practice-changing," Dr. Leena Gandhi, an associate professor at NYU Langone Health, said of Merck's study.

That alphabet soup hints at the characterization of patients in Bristol's trial: It focused on those with more than 10 mutations observed in their cancer, a status referred to as high tumor mutational burden, or TMB. NSCLC is short for non-small cell lung cancer, which is the most common form of the disease — accounting for 85 percent of cases. Patients with high TMB account for about 45 percent of patients with non-small cell lung cancer, Hellmann said.

Both drugs were tested in patients who hadn't yet tried other therapies, but whose cancer had already spread.

Lung cancer is a huge area of interest both for medicine and for Wall Street because of the sheer size of the problem: Though it's the second-most-common form of cancer in both men and women, it causes by far the most cancer-related deaths. More than 234,000 Americans are expected to be diagnosed with lung cancer this year, and it's expected to cause more than 154,000 deaths.

That also means it's a large market, estimated by investors at $10 billion to $15 billion at its peak potential, according to Evercore ISI analyst Umer Raffat.

So even though Merck's Keytruda and Bristol's Opdivo are already multibillion-dollar drugs, these results could have important implications for their market share.

Merck didn't focus on tumor mutational burden in its trial; it separated patients into groups based on their cancers' level of expression of a marker called PD-L1 but said the combination with Keytruda worked better than chemo alone regardless of patients' PD-L1 status.

The trial focused on a measure known as overall survival — how long patients live — as well as one known as progression-free survival, or how long patients live without their cancer growing.

After a median follow-up of 10.5 months, patients in the chemo-only arm of Merck's trial lived for a median of 11.3 months. Those taking Keytruda along with chemo lived long enough so that a median overall survival duration wasn't met after the 10.5-month follow-up.

"People clearly are living longer," Merck Research Laboratories President Dr. Roger Perlmutter said in an interview. "That's a very dramatic result."

Bristol-Myers also said the benefit conferred by its drug combination over chemotherapy was observed regardless of PD-L1 status. Its results focused on progression-free survival, following patients for a minimum of 11.5 months. It said overall survival data "are still maturing."

Safety and cost are also important considerations for the medicines; Merck's Keytruda showed a safety profile consistent with previous studies, such as nausea, anemia and fatigue, but also was linked to a higher rate of acute kidney injury, Gandhi said.

Merck may have the advantage when it comes to cost, according to Cowen, which noted chemotherapy as the combination drug costs less than a second immunotherapy medicine. Keytruda and Opdivo alone are both priced at about $150,000 a year; Yervoy can add about $100,000 to the Bristol combination's price tag.